Support Heroku CI

[runesoerensen]

This PR adds support for Heroku CI, implementing the Testpack API by:

• Moving common compilation logic from bin/compile to lib/compile_common.sh.
• Updating the CNB GitHub action to read and update the pinned CNB version from lib/compile_common.sh instead of bin/compile.
• Adapting the process_env_files function to take a "scope" parameter. This allows setting the launch scope when invoked from the bin/compile script (retaining the current behavior), and the build scope when invoked from the new bin/test-compile script (so bin/test can use the build-only sdk and nuget layers).
• Checking if BUILD_CONFIGURATION or MSBUILD_VERBOSITY_LEVEL env vars are set in bin/test, and passing their values to the dotnet test command if present. While it's somewhat unlikely that users will configure these in their Heroku CI environment, the bin/test-compile script will detect these values (as they're support by the CNB), so we should support them in bin/test as well.

It's worth noting that the bin/test-compile script will invoke the CNB's bin/build executable and publishes the app (like bin/compile), which is not necessary to run bin/test. We may want to add logic to the CNB that allows skipping the dotnet publish step to reduce the bin/test-compile execution time.